284 



PLANTS 



the earth. 



307. Germination of a Bean. When we put a seed into 

 the ground under proper conditions, it responds to the 

 stimuli it receives, by germinating. Germinating means 

 all the changes in the contents of the seed until the tiny 

 plant in the seed has become an independent plant, or 

 seedling. The bean gives us a convenient seed for study. 

 If it is soaked in slightly warmed water, say, over night, 

 the shriveled surface becomes distended, or filled, by the 

 water, which passes, by diffusion, through the "skin" 

 (cf. 106). If the bean is then placed in a box of earth, 

 set in a sunny window, and the earth is kept moist, growth 

 will begin. In a few days a curved "stem" will push up 

 When this straightens itself, it raises into the 

 air the two halves of the bean; between 

 them there is a pair of tiny leaves. At 

 the lower end of the small stem roots are 

 developed. As the bean seedling becomes 

 older, it forms pair after pair of true 

 leaves, always from a growing "tip"; the 

 "halves" of the bean become thinner, turn 

 green, and finally fall off. The plant is 

 now independent (Fig. 232). 



What are the parts of the original 

 seed, and what change do they undergo in 

 germinating? We can answer these ques- 

 tions by studying the soaked bean before 

 planting. We find that the "halves" are enclosed in a 

 tough "skin" ; this is the testa. All inside the testa is the 

 embryo. The "halves" are called cotyledons, or seed 

 leaves. They contain food for the embryo. It is for 

 this food that we eat the bean. Between the cotyledons 



FIG. 232. A Bean 

 Seedling, with 

 Its Roots, Stem, 

 Cotyledons, First 

 Pair of True 

 Leaves, and Leaf 

 Bud. 



